Stable carbon isotope compositions of isoprenoid chromans in Cenozoic saline lacustrine source rocks from the Western Qaidam Basin,NW China: Source implications |
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Authors: | YongDong Zhang AiZhu Jiang YongGe Sun LiuJuan Xie PingXia Chai |
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Institution: | [1]State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China [2]Department of Earth Science, Zhejiang University, Hangzhou 310027, China [3]Graduate University, Chinese Academy of Sciences, Beijing 100049, China |
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Abstract: | Although the distribution of mono-, di- and trimethylated 2-methyl-2-(4,8,12-Trimethyltrideeyl) chromans (MTTCs) is well understood
as an indicator of water salinity, their origin and formation mechanism are still ambiguous and under debate. In this paper,
abnormally high levels of MTTCs were detected in Cenozoic saline lacustrine source rocks from the Western Qaidam Basin. Using
a two-step column chromatography method, the MTTCs and naphthalenes were separated from other aromatic compounds and concentrated
in one fraction, so that the stable carbon isotope compositions of these compounds could be accurately measured. Similar carbon
isotope ratios for the mono-, di- and trimethylated 2-methyl-2-(4,8,12-Trimethyltrideeyl) chromans in a given sample suggest
the MTTCs may share the same biological source(s). The MTTCs from the Western Qaidam Basin have similar carbon isotope compositions
to primary producer-derived pristane and phytane. However, the δ
13C values for the MTTCs showed significant differences with other primary producers-derived biomarkers (e.g. C27 and C29 steranes), probably indicating a distinct microalgae source from specific niches for the MTTC compounds. We speculate that
the MTTCs distribution patterns may be controlled by the water chemistry dynamics in niches, which, in turn, is affected by
hypersaline bottom water. |
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Keywords: | saline lacustrine source rocks MTTCs stable carbon isotope composition microalgae Qaidam Basin |
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